Optical communication systems employing wavelength division multiplexed (WDM) technology achieve large transmission capacity by spacing optical channels as closely as possible, typically less than a nanometer (nm) apart. As the channel spacing decreases, monitoring spectral characteristics of the channels becomes more critical in verifying system functionality, identifying performance drift, and isolating system faults. For example, such monitoring is critical in detecting wavelength drift, which can readily cause signals from one optical channel to cross into another. Also, real-time feedback to network elements is critical to ensure stable operation of optical amplifiers commonly employed in the network.
Optical instruments for measuring optical power as a function of wavelength, called optical spectrum analyzers (OSAs), are known in the art. Most conventional OSAs use a wavelength tunable optical filter, such as a Fabry-Perot interferometer or diffraction grating, to resolve the individual spectral components. In the latter case, light is reflected off the diffraction grating at an angle proportional to the wavelength. The spectrum of the light is then analyzed on the basis of the angle at which the light is diffracted using a detector array. Alternatively, the diffracted light is moved over a slit and then detected using a small detector.
Traditional optical spectrum analyzers (OSAs) are manufactured as laboratory devices which operate under laboratorial environmental conditions. A sophisticated wavelength and optical power calibration from time to time is required to ensure the wavelength and power accuracy of the device. Furthermore, they are generally bulky as well as costly.
Optical communication systems require industrial grade optical performance monitors (OPM), which function similarly to the traditional OSA, but are however subject to stringent industrial requirements. They must be relatively inexpensive, compact in size, with the reporting power and wavelength accuracy nearly the same as laboratorial grade OSAs, however without requiring extra calibration during the lifetime of the device, and be capable of monitoring light at densely spaced frequency points with high wavelength resolution and high dynamic range.
It is advantageous to have an OPM capable of monitoring all channels in one optical band of an optical communication link. It is also advantageous to have an additional functionality of monitoring an optical to signal noise ratio (OSNR) for each channel, which requires monitoring not only individual channels, but also light between channels to estimate an optical noise level, thereby further increasing spectral resolution requirements for an OPM. Today's WDM networks may employ as many as 200 channels with 25 GHz spacing between the channels in one optical communication band of approximately 5000 GHz; these networks would benefit from an OPM capable of monitoring at lest 200 frequency channels with 25 GHz spacing. Such an OPM could also be advantageously used in communication systems having 200 GHz, 100 GHz, and 50 GHz spaced channels by providing an OSNR monitoring capability.
One type of industrial-grade OPMs acquires all monitored spectral points of an optical spectrum of an input signal in parallel by dispersing the input light in space and using a plurality of photodetectors, e.g. a photodiode array (PDA), to simultaneously acquire spectral information at a plurality of monitored frequencies; a bulk grating, a blazed fiber Bragg grating, a waveguide echelle grating or an array waveguide grating (AWG) can be used as such a dispersive element.
Disadvantageously, both the size of the dispersive element and the number of photodiodes in the PDA scale proportionally to the wavelength resolution, thereby increasing the size and cost of the device and reducing it reliability. If the OSNR of each channel is to be measured, several photodetectors have to be provided within the dispersed light of a single channel. Thus, a four channel optical monitor typically requires at least 12 photodiodes. Since current photodiode arrays are typically supplied in strips of up to 128 photodiodes, this allows monitoring of just over 30 channels.
For example, U.S. Pat. No. 5,617,234 issued Apr. 1, 1997 in the name of Koga et al. discloses a multi-wavelength simultaneous monitoring circuit capable of precise discrimination of wavelengths of a WDM signal, and suitable for optical integrated circuits. The device proposed by Koga is an AWG that has a single input port and multiple output ports and has photodetectors coupled to the output ports of the AWG. An AWG has a functionality of splitting an input signal into several output frequency bands, each having a bandwidth b, centered at a set of frequencies fn spaced by an output frequency spacing Δf≧b, and dispersing them in space to different locations where they are picked up by output waveguide to be output through their respective output ports. Koga's device requires an AWG having a number of output ports equal to a number of monitoring channels with frequency resolution better than spacing between the channels, and a number of costly photodetectors equal to the number of channels to be monitored, without providing an OSNR measurement capability.
AWGs offer several advantages when used as the dispersive elements, such as relative compactness, option of on-chip integration with other optical components of an optical circuit thereby drastically lessening optical losses and reducing cost and complexity of the optical circuit, and manufacturing technology amenable to mass-production. However, they typically offer only limited frequency resolution with a limited number of output channels, typically between 8 and 40, with a frequency spacing between output channels ranging from 400 to 50 GHz. Decreasing the frequency spacing further below 25 GHz as may be necessary for accurate OSNR monitoring requires progressively larger and more expensive devices, with increasing cost per monitored channel.
Another known type of OPMs involves sequential acquisition of spectral data points, and is based on tunable filters with output coupled to a typically single photodetector, wherein the spectrum is measured by scanning the filter passband over a frequency range of interest, and adjacent spectral points are accessed sequentially in time. The tunable filter employed in this approach can be based on a bulk—surface or volume—grating, a fiber Bragg grating, a tunable linear or ring resonator. However, using tunable filters may require complex dynamic control loops and real-time monitoring of the tuning to ensure reproducibility. Higher wavelength resolution requires larger tunable filters and progressively more strict requirements on tuning filters wherein progressively finer tuning is required, complicating the control loops and affecting reproducibility issues.
Recently, attempts have been made to provide a solution to this problem of scaling by combining the aforedescribed sequential and parallel acquisition approaches in a way wherein the size, the design complexity, e.g. the number of photosensitive elements, and the control complexity of the monitor scales sub-linearly with a number of monitored wavelengths within a monitored range of wavelengths.
A U.S. Pat. No. 6,915,030 to Svilans et al. assigned to JDS Uniphase, the assignee of this application, discloses an AWG-based OPM that combines a single-input port AWG with a tunable filter having a bandwidth and an FSR to obviate the aforementioned problems, by monitoring a larger number of channels, greater than a number of AWG output ports and associated photodiodes. The tunable filter pre-selects periodic subsets of channels to be input through the single input port of the AWG, and different subsets of channels are sent sequentially to the input port of the AWG thereby time-sharing the AWG and associated photodiodes coupled to the output ports thereof to acquire information about a number of spectrally-resolved channels larger than the number of AWG output ports and coupled to them photodiodes. Although the devices described by Svilans et al. offer considerable advantages by reducing the number of photodiodes per WDM channel, it employs a tunable filter that may require real-time monitoring and relatively complex control circuitry to ensure wavelength tuning reproducibility.
U.S. Pat. No. 6,753,958 in the names of Berolo et al discloses an alternative approach to monitoring of a large number of wavelength with a relatively small number of photodiodes without dynamically tuning frequency-selective elements that may require complex real-time monitoring and control. Berolo et al teach an OPM that has an optical input port coupled through a switch to a plurality of input waveguides, which are sequentially switched to provide light received from the input port via one of the input waveguides to a waveguide echelle grating, which disperses the light toward a plurality of photodetector. The echelle grating disperses light received from an input waveguide in dependence upon the input waveguide position, so that light picked up by the photodetectors has different centre wavelengths depending on via which of the input waveguides the light entered the grating. By arranging the input waveguides so that the centre wavelengths sampled by photodiodes shift by a fraction of the channel spacing of the WDM signal when the light is switched between adjacent input waveguides, the WDM signal carried by the light can be sampled with a frequency period equal to the fraction of the channel spacing.
The method of Berolo et al may enable monitoring a WDM signal at a number of wavelength equal to the number of photodiodes times the number of input ports in a compact relatively inexpensive device. However, the particular approach of Berolo et al wherein spectrally adjacent central wavelengths shifted by a small fraction of the channel spacing equal to the OPM resolution are sampled by coupling the light into adjacent input waveguides may be severely limiting at least in some applications, since it may require the input waveguides to be positioned close to each other when the required spectral resolution is small, as for the OSNR measurements, which may lead to an undesirable optical coupling between adjacent input waveguides.
An object of this invention is to provide an optical performance monitor having a dispersive element with multiple output ports and switchable multiple input ports configured for accurately measuring the OSNR of individual WDM channels of a multi-channel optical signal without unwanted optical coupling between the input ports.
It is another object of this invention to provide a method of accurate OSNR measurements for multiple optical channels of a WDM optical signal in an optical performance monitor, wherein the optical spectrum of the WDM signal is sampled at a plurality of sampling frequencies by switching between input ports of a multi-input multi-output AWG.
Another object of this invention is to provide an optical performance monitor and a method of using thereof for accurate simultaneous OSNR measurements for multiple optical channels of a WDM optical signal, wherein an optical spectrum of the WDM signal is sampled at a plurality of sampling frequencies by switching between input ports of a multi-input multi-output AWG, and wherein spectral samples at adjacent sampling frequencies are acquired by switching between non-adjacent input ports.